Sorry if me message there sounded short, that was not my intention. I simply wanted to inform you of the plugin that would do exactly what you were asking.
There is some documentation on how to use the location plugin too, for the future.
Mind you, you can do this is “Central GUI Server->Storage Nodes about” layout, or really however you feel appropriate for your environment.
Location plugin will likely get an overhaul in the next few years (yes years) with more automating of how to select which storage node’s based on subnets/ip ranges, but for now it’s completely manual (and thus extremely granular too).
https://wiki.fogproject.org/wiki/index.php?title=Location_Plugin